1. Field of the Invention
This invention relates to a switched reluctance drive system. In particular, it relates to a switched reluctance drive system that is configured to draw current at a high power factor from an electrical supply.
2. Description of Related Art
The characteristics and operation of switched reluctance machines are well known in the art and are described in, for example, "The Characteristics, Design and Application of Switched Reluctance Motors and Drives" by Stephenson and Blake, PCIM '93, Nurnberg, Jun. 21-24, 1993, which is incorporated herein by reference.
FIG. 1 shows a typical switched reluctance drive in schematic form arranged to drive a load 19. The drive comprises a switched reluctance motor 12 having a stator and a rotor, a power converter 13 and an electronic control unit 14. The drive is supplied from a DC power supply 11 that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14.
FIG. 2 shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16. In this circuit, a switch 21 is connected between the positive terminal of a power line and one end of the winding 16. Connected between the other end of the winding 16 and the negative terminal of the power supply is another switch 22. Between switch 22 and the winding 16 is connected the anode of a diode 23, the cathode of which is connected to the positive line of the power supply. Between switch 21 and the winding 16 is connected the cathode of another diode 24, which is connected at its anode to the negative line of the power supply. Switches 21 and 22 act to couple and de-couple the phase winding 16 to the source of DC power, so that the winding 16 can be energized or de-energized.
Many other configurations of switching circuitry are known in the art, some of which are discussed in the Stephenson & Blake paper cited above.
For proper operation of the drive, the switching must be correctly synchronized to the angle of rotation of the rotor. A rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms. For example it may take the form of hardware, as shown schematically in FIG. 1, or of a software algorithm that calculates the position from other monitored parameters of the drive system, as described in e.g. European Patent Application No. EP-A-0573198 (Ray), incorporated herein by reference. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required.
The switched reluctance drive is essentially a variable speed system characterized by voltages and currents in the phase windings 16 that are quite different from those found in traditional machines. FIG. 3(a) shows a typical voltage waveform applied by the controller to the phase winding 16. At a predetermined rotor angle, the voltage is applied by switching on the switches 21 and 22 in the power converter 13 and applying a constant voltage for a given conduction angle .theta..sub.c. The current rises from zero, typically reaches a peak and falls slightly as shown in FIG. 3(b). When .theta..sub.c has been traversed, the switches in the power converter 13 are opened and the action of the energy return diodes 23 and 24 places a negative voltage across the winding, causing the flux in the machine, and hence the current, to decay to zero. There is then a period of zero current until the cycle is repeated. It will be clear that the phase draws energy from the supply during .theta..sub.c and returns a smaller amount to the supply thereafter. It follows that the supply, shown as 11 in FIG. 1, needs to be a low-impedance source that is capable of receiving returned energy for part of its operating cycle. FIG. 3(c) shows the current that is supplied to the phase winding 16 by the power converter 13 during the period of energy supply and the current that flows back to the converter 13 during the period of energy return.
Typically, the DC power supply 11 of FIG. 1 is realized by rectifying the AC mains supply, as shown in FIG. 4 where the mains supply 30 is shown as an AC voltage source 32 in series with a source impedance 34. In most cases, the impedance 34 is mainly inductive. This inductance can be increased by adding further inductive components in series. A rectifier bridge 36 is provided having four terminals A, B, C and D, two of which, A and C, are connected to the mains supply 30, the other two, B and D, being connected across a capacitor 38. The rectifier bridge 36 rectifies the sinusoidal voltage of the source and the output voltage is smoothed by the capacitor 38. Connected in parallel with the capacitor 38 and the rectifier bridge 36 is a switched reluctance drive 39 (shown schematically), typically comprising the blocks 12, 13 and 14 of FIG. 1.
The lines marked +V and -V in FIG. 4 are generally known as the DC link, and capacitor 38 as the DC link capacitor.
In the absence of any load on the DC link, the capacitor 38 is charged up by successive cycles of voltage to the peak voltage of the sinusoidal supply 30. The capacitor 38 must therefore be rated for at least the peak of the supply voltage. As resistive load is applied, and when the supply voltage is below the capacitor voltage, energy is drawn from the capacitor 38. When the rectified supply voltage rises above the capacitor voltage, the capacitor 38 is charged up.
The size of the capacitor 38 and the amount of current drawn by the load interact. Generally, the capacitor is sized so that there is a relatively small amount of droop on the DC link voltage while the capacitor is supplying the load. FIG. 5 shows the rectified voltage and the DC link voltage for a typically sized capacitor, from which it can be seen that the DC link voltage is held approximately constant. The shape of the current from the supply is complex, since it is dependent not only on the size of the DC link capacitor but also on the size and nature of the source impedance. If the capacitor 38 is very large (so that the voltage ripple is effectively zero) and the supply impedance is negligible, the plot of current vs time has a very large spike centered on the peak of a like plot of the rectified voltage waveform. In practice, some supply impedance is always present and has the effect of widening the width of the current pulse and hence reducing its magnitude. Nevertheless, the rectifier must be rated to carry the high peak current.
The general form of the supply current as a function of time is shown in FIG. 5, where it should be noted that the current is zero for a significant fraction of the overall cycle. This has an undesirable effect on the power factor of the overall circuit. Power factor is defined as the ratio of the real power supplied to the load to the apparent power (i.e. the volt-amperes) supplied to the circuit. With low supply impedance, the power factor is typically around 0.5. With inductance added to the supply it is possible to increase the width of the current pulse and hence increase the power factor, but a value of around 0.65 is generally considered to be the practical and cost-effective limit.
These low power factors can cause problems for the designers of electrical equipment, for two reasons. Firstly, the supply may have a minimum limit on the power factor that can be drawn, in which case the power factor has to be corrected by some other means. Secondly, for appliances operating from domestic power supply outlets, there is a fixed current limit: for example in the US, domestic supplies are often limited to 15A at 120V. This allows a nominal power of 1800W to be drawn at unity power factor, but proportionally less at reduced power factors (typically 1000W using the circuit of FIG. 4). For these reasons, power factor correction (PFC) circuits have been developed to raise the power factor of a given load. European Patent Application No. EP-A-0805548 (Sugden), incorporated herein by reference, describes various active power factor correction circuits. These are known as "active" circuits because they typically use a switch placed across the output of the rectifier to modulate the current drawn from the supply and force it to follow the phase and waveshape of the supply voltage. However, while these circuits can greatly improve the power factor, they are expensive and bulky. A cheaper and smaller circuit is required, particularly for domestic appliances.
In addition to active PFC circuits, passive PFC circuits are known. These do not use active switches but employ combinations of passive components to improve the power factor. One such circuit is described in "Improved Valley-Fill Passive Power Factor Correction Current Shaper Approaches IEC Specification Limits", PCIM Journal, Feb. 1998, pp. 42-51, Sum, KK, which is incorporated herein by reference. This circuit is shown in FIG. 6, and includes the supply 30 and rectifier bridge 36 described with reference to FIG. 4. However, in this case, connected across terminals B and D of the rectifier bridge 36, there is a series combination comprising a capacitor C1 connected to the anode of a diode D3 that is connected via its cathode to another capacitor C2. Connected between capacitor C1 and diode D3 is the cathode of another diode D1, the anode of which is connected to the -V line of the DC link. Connected between capacitor C2 and diode D3 is the anode of yet another diode D2, the cathode of which is connected to the +V line of the DC link.
When the supply voltage of the circuit of FIG. 6 reaches its peak value, charging current is able to flow through rectifier 36 into the series connection of C1, D3 and C2. The capacitors are each rated at half the peak of the rectified voltage. When a resistive load R1 is applied, the action of D1 and D2 is to connect C1 and C2 in parallel, so that when the rectified voltage falls to half its peak value, the load is supplied from the two capacitors. When the rectified voltage is above half its peak value, the load is supplied directly from the rectifier. FIG. 7 shows the voltage waveforms. The circuit thus fills in part of the trough or valley between the pulses of voltage, hence the name "valley-fill".
Assuming that the capacitors are fully charged by the peak portion of the voltage, they start to supply current when the supply voltage falls to half its peak, i.e. at 150.degree.. Neglecting any droop in the capacitor voltage, they cease to supply current at the next value of half peak voltage, i.e. at 30.degree., when D1 and D2 become reverse biased. Between these angles, the current for the load is supplied entirely from the rectifier 36. If the capacitors have little voltage droop, their charging current is centered round the peak voltage, giving the composite current waveform shown in FIG. 7. In practice, however, some droop is accepted to gain economy in capacitor size, so the charging spike is spread out and the rectifier 36 conducts earlier than 30.degree..
Two things should be noted about the circuit of FIG. 6. Firstly the supply current is better spread than for the traditional circuit of FIG. 4, and consequently has a lower harmonic content. This leads to improved power factor. Secondly, the capacitors C1 and C2 are rated only for half the supply voltage and they supply load current only during the "valley". This permits smaller capacitors to be used and leads to an economical circuit.
In the paper by Sum cited above, it is explained that the circuit of FIG. 6 is good for improving power factor, but is only suitable for small, resistive loads that do not return energy to the supply (ibid, p. 44). As an example, Sum describes how the basic circuit can be adapted for use with fluorescent lighting loads by adding a voltage doubler. This improves the power factor further but at the expense of efficiency. This is inappropriate to a switched reluctance drive, where high efficiency of the power conditioning circuits is essential.
For switched loads (resistive or inductive) where the switching frequency is different to the mains supply frequency, the valley-fill circuit of FIG. 6 is regarded as being of little value. This is because of the presence of inductance in the supply impedance, which forces the capacitor voltage to rise whenever the current to the load is interrupted. As there is no guarantee that sufficient charge is taken out of the capacitors during the "valley fill" period, this mechanism can lead to excessive capacitor voltage and eventual capacitor failure. While the rise in capacitor voltage might be accommodated in a very small drive, increasing the capacitor size in a larger drive to overcome the problem defeats the object of achieving a low-cost, efficient circuit.
As mentioned above, a switched reluctance drive is an inductive switched load that returns energy to the supply circuit during part of each operating cycle. If operated using traditional methods of control and coupled with the circuit of FIG. 6, this returned energy adds to the previously described problem with supply inductance to further stress the capacitors. With the traditional supply circuit of FIG. 4, this is not a problem since, although the rectifier is not receptive to returned energy, the DC link capacitor is typically large enough to absorb the energy without a problem. Although the valley-fill circuit is attractive in the sense that the power factor is potentially improved, the small capacitors associated with it cannot cope with the returned energy from the machine as well as the energy from the supply inductance, as described above. There is, therefore, a need for a PFC circuit that can operate successfully with a switched reluctance drive.